Investigation of Fluid Migration Pathways in the Shallow Subsurface of the Great South Basin, through the use of High-Resolution Seismic Imaging of Fault and Fracture Systems

The Great South Basin is one of New Zealand's largest undeveloped offshore basins and is known to have significant oil and gas prospects. The Great South Basin lies offshore from the coast of Southland and South Otago, and at over 85,000 km2 is one of New Zealand’s largest petroleum basins. Understanding the "plumbing" of the Great South Basin through seismic imaging of fault and fracture systems provides information on oil and gas migration pathways and success of seals and traps within the basin. Reprocessing of high-resolution industry seismic data has improved images of the shallow sea floor. Interpretations of these reprocessed lines provide basic information on the morphology of the Toroa Dome Reef Complex. Further reprocessing, including near-trace and far-trace stacks, examinations of amplitude and phase change, FX-space deconvolution and filtering of noise has helped to confirm that the reefs have variable size and structure but in all cases are much harder than the surrounding seabed. The reflectivity contrast of the seafloor reefs relative to the overlying water column is so great that wipe out occurs below the reef, obscuring internal structures. The reefs are interpreted to be volcanic in origin. However the possibility that the reefs consist of methane-derived carbonate is possible. A mud volcano origin has been discounted. Future work based around high frequency seismic surveys and a dredge sampling program could solve remaining uncertainties about the reefs' origins